1. Field of the Invention
The present invention relates to a hydraulic brake actuator for use in a braking force control system of an automotive vehicle.
2. Description of the Related Art
This type of brake actuator is generally interposed in a hydraulic circuit connecting a master cylinder to be operated by a driver of an automotive vehicle and a wheel cylinder associated with a vehicle wheel, for controlling the braking fluid pressure to be applied to the wheel cylinder.
The applicant has already proposed a hydraulic brake actuator which is capable of performing anti-skid control function and/or traction control function, and which can be designed to avoid a bulky overall arrangement of the actuator and, in particular, to have a compact arrangement of the piston drive section, such as an electric motor. Such a brake actuator is disclosed, e.g. in Japanese Patent Application No. 2-275,260 which has not been laid-open for public inspection as of the priority date of this application.
With the brake actuator according to the above-mentioned proposal, as for the anti-skid control function, when the wheel of an automotive vehicle exhibits a slipping tendency due to an excessive braking force, two valves are controlled in accordance with information relating to the slipping condition. That is, an electromagnetically operated first valve is opened to interrupt the communication between the master cylinder and two chambers which are defined by a piston in a control cylinder. Subsequently, a similarly electromagnetically operated second valve is closed to interrupt the communication between the two chambers of the control cylinder and maintain these chambers at the wheel cylinder pressure at the instance of occurrence of the slipping. Then, the piston is applied with a thrust by a driving motor to increase the volume of the chamber in the control cylinder on the side of the wheel cylinder and decrease the wheel cylinder pressure, for performing the desired anti-skid control function and suppressing the wheel slipping.
The driving force required for driving the piston corresponds to the force used to reduce the pressure within the chamber of the control cylinder on the side of the wheel cylinder, from the pressure value at the instance of occurrence of slipping to a lower pressure value which allows suppression of the slipping so that it is possible to minimize the capacity of the driving motor.
On the other hand, as for the traction control function which is performed upon occurrence of a wheel spinning due to an excessive driving force, only the second electromagnetically operated valve is closed and the piston is applied with a thrust so as to decrease the volume of the chamber in the control cylinder on the side of the wheel cylinder and increase the wheel cylinder pressure, for applying an increased braking force to the wheel and performing the desired traction control function.
The above-mentioned arrangement according to the applicant's earlier proposal proved to be highly advantageous in that a single actuator could be used to perform the anti-skid control function and in addition the traction control function, with a compact drive motor for the piston. Nevertheless, this brake actuator could be further improved without the sacrifice of the above-mentioned advantages.
The anti-skid control function, for example, requires two electromagnetic driver devices for the two valves and one driver device for applying the thrust to the piston, whereas it is desirable to reduce the total number of driver devices and minimize the size, weight, energy consumption, manufacturing cost, etc. Particularly, when it is required to provide a brake actuator for each wheel of an automobile vehicle, as is the case in a 4-channel anti-skid control and/or traction control for a 4WD vehicle, it is highly desirable to further simplify the overall arrangement and lower the manufacturing cost of the individual actuator while simultaneously maintaining the above-mentioned advantages of the arrangement according to the applicant's earlier proposal.
SUMMARY OF THE INVENTION
It is a primary object of the present invention to provide an improved hydraulic brake actuator which makes it possible to simplify the overall arrangement and lower the manufacturing cost.
Briefly stated, the present invention provides a hydraulic brake actuator for use in a brake system of an automotive vehicle, which includes a master cylinder to be operated by a driver and a wheel cylinder connected to the master cylinder through a hydraulic circuit for applying a braking force to a wheel.
The brake actuator according to the present invention is interposed in the hydraulic circuit and comprises a control cylinder device having a chamber which is connected to the wheel cylinder and which can be selectively connected to the master cylinder and disconnected therefrom, and a piston axially displaceably accommodated in the chamber for changing the volume of the chamber.
The brake actuator according to the present invention further comprises a piston driver device for causing an axial displacement of the piston, and a valve device having an inlet port connected to the master cylinder and an outlet port connected to the chamber, and operatively connected to the piston of the control cylinder device so as to be opened or closed in response to the displacement of the piston.
With the brake actuator according to the present invention, it is possible to perform a desired anti-skid control function and in addition a traction control function, essentially in the same manner as the brake actuator according to the applicant's earlier proposal as mentioned above. On this occasion, the axial displacement of the piston of the control cylinder device is induced by the piston driver device. Such displacement of the piston results not only in change in the volume of the chamber within the control cylinder device, which is connected to the wheel cylinder, but also in the opening or closure of the valve device which is interposed between the master cylinder and the chamber within the control cylinder device, since the valve device is operatively connected to the piston.
The valve device according to the present invention does not by itself require a solenoid or the like electromagnetic driver device, so that it is possible to simplify the overall arrangement of the brake actuator and minimize the size, weight, energy consumption, manufacturing cost, etc., and to carry out the desired control with an improved response characteristic.
The present invention will be explained in further detail hereinafter, by referring to some preferred embodiments shown in the accompanying drawings .